Individualizing the locoregional management of breast cancer is challenging for both patients and their clinicians. The acceleratetd pace of introduction and adoption of evaluative tests to inform management of breast cancer may facilitate individualization, but the patterns of use and impact of these tests is not sufficiently understood. Standard treatments include breast conserving surgery (BCS) with whole breast radiation vs mastectomy, which have been shown to yield equivalent clinical outcomes in mature randomized trials. However, patients may now also receive more aggressive approaches, including contralateral prophylactic mastectomy, or less aggressive approaches, such as BCS with partial breast irradiation (PBI) or omission of axillary lymph node dissection (ALND) for limited metastases on sentinel node biopsy. The health benefits of the rapidly evolving approaches for breast cancer locoregional management cannot be maximized without ongoing population-based research to track and evaluate how these tests and treatments are used to better individualize care. Individualized care in this context is achieved when 1) evaluative tests are ordered in clinically appropriate patients, 2) treatment receipt is largely driven by evidence-based clinical indications that address expected benefit in terms of disease-free and overall survival, and 3) patients are informed, satisfied with the process, and their preferences and values are incorporated into decisions. The project proposed here is a population-based study of patients newly diagnosed with breast cancer and reported to the SEER registries of New Jersey and Georgia, as well as their surgeons. Aim 1 is to examine the use of newer evaluative tests (breast MRI and genetic testing for BRCA1/2 mutation) for patients diagnosed with breast cancer and the patient and clinician correlates of testing. Aim 2 is to examine the receipt of contralateral prophylactic mastectomy, omission of ALND, and PBI, and the patient and clinician correlates of uptake of these emerging approaches for locoregional management of breast cancer. As part of this aim, we will evaluate the effects of newer evaluative tests on receipt of treatments. Aim 3 is to explore the individualization of decision-making by evaluating the extent to which patients are informed about tests and treatments and are satisfied with the decision-making process.